8 notebooks cost $10.56. Which equation would help determine the cost of 9 notebooks?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 9 notebooks. Since 9 notebooks cost $x$ , we have the following proportion: $\dfrac{9}{x}$ We can write the fact that 8 notebooks cost $10.56 as a proportion: $\dfrac{8}{\$10.56}$ The cost changes along with the number of notebooks purchased, and so the two proportions are equivalent. $\dfrac{9}{x} = \dfrac{8}{\$10.56}$